The present invention relates to an image forming apparatus wherein a sheet stacked on a sheet feed tray is fed and an image is formed on the sheet.
Image forming apparatuses such as a copying machine, a printer and facsimile machine are introduced in many offices, and a higher speed of the image forming apparatus and higher image quality are demanded by users. In recent years, electrophotographic image forming apparatuses are introduced also in the print industry, and a higher speed and higher image quality are demanded by users.
As an art to realize higher image quality, there is a technology in which physical properties of each sheet fed out of a sheet feed tray including, for example, a width and a whiteness degree, or, positions such as deviations, for example, are detected to correct image data, and an image is formed on the sheet based on the corrected image data.
In the technology described in Unexamined Japanese Patent Publication Application No. 2005-41582, for example, a sheet deviation sensor is provided on a conveyance path for sheets, then, image data are shifted in the main scanning direction on the memory to be corrected based on the results of the detection by the sensor, and an image is formed on a sheet based on the corrected image data. This technology makes it possible to take a deviation of each sheet into consideration and thereby to form a high image quality that is constantly at a fixed position from an edge of the sheet.
For forming an image by considering physical properties and positions of each sheet fed out of a sheet feed tray, it is necessary to detect a sheet that is fed from sheet feed tray 1000 by detection sensor 3000 installed on conveyance path 2000 as shown in FIG. 10, for example, and to correct image data based on results of the detection. Further, when realizing a higher speed while conducting correction of image data, it is necessary to feed sheets continuously from sheet feed tray 1000 by shortening an conveyance interval of sheets, and to detect physical properties and positions of each sheet with detection sensor 3000.
Since a certain period of time is needed from the moment when physical properties and others of each sheet are detected by the detection sensor 3000 to the moment when an image formed on photoreceptor 4000A arrives at transfer section 5000 after image data are corrected, it is necessary to detect physical properties and others of a sheet early, and to form images on photoconductor 4000A based on results of the detection in succession, for forming images on sheets fed continuously at short intervals.
When taking this point into consideration, it is necessary to arrange the detection sensor 3000 at the upstream side of the conveyance path 2000, and to make a length of a conveyance path between the detection sensor 3000 and the transfer section 5000 to be as long as possible so that a plurality of sheets may stay between the detection sensor 3000 and the transfer section 5000. In this way, physical properties of sheets can be detected early, and a higher speed can be realized.
However, if the conveyance path between the detection sensor 3000 and the transfer section 5000 is made to be longer linearly, an apparatus needs to be large in size. In addition, if the conveyance path between the detection sensor 3000 and the transfer section 5000 is deflected to be made longer for preventing a large-sized apparatus, removal of jammed sheets is difficult when a sheet jam takes place at the deflected portion.